|Oak savannas are
fire-dependent ecosystems and fire is an essential element in
their establishment and management. There are two kinds of fires
in ecosystems: wildfires and prescribed fires. Wildfires are
those that have started spontaneously, generally by lightning.
Prescribed fires are those set by land managers to bring about
desirable changes in ecosystems and are based on a written plan,
the burn prescription. We are dealing here only with prescribed
Fire plays several important roles.
• Removes oak leaves and litter, opening up the soil so that
plants can grow faster. This also permits planted seeds to reach
• Helps perpetuate fire-dependent species.
• Helps in control of harmful insects or diseases.
• Improves wildlife habitat.
• Enhances the appearance of the site and increases the scenic
• Helps improve access to the savanna, making it easier to walk
the property and survey the ecosystem.
• Top-kills woody vegetation, shrubs and small trees, but does
not kill the oaks. Top-killing does not eliminate the undesirable
woody plants, but sets them back.
• Kills invasive conifers such as red cedar.
• Top-kills brambles.
• Consumes downed brush and branches, making it possible for
fires to carry better.
• Hazardous fuel reduction.
• Recycles nutrients from the litter into the soil.
Fire is one of the most cost-effective ways of maintaining
a restored savanna, but should always be used as part of an
integrated management system. Fire should never be used by
itself. Also, fire is not a substitute for brush removal.
In fact, it is undesirable and counterproductive to burn an
unrestored savanna, because fire does not eradicate brush.
Burns should only be conducted after the initial major restoration
work has been completed.
Is fire in the woods harmful?
The public has been conditioned by decades of information
about Smokey the Bear to believe that fire should never be
allowed in a woods. This is incorrect as far as oak woods
are concerned, whether in savannas or woodlands.
Thus, when burning a wooded area, it is important to educate
the public about its importance. Fire in an oak savanna is
not bad, but good, indeed vital to the health of the savanna.
Because of the low flame heights, fire in an oak savanna is
low-intensity prescribed burn in a bur oak savanna.
Adaptation of oaks to fire
The following characteristics of oaks make them especially
resistant to fire, relative to other hardwood species.
• Thick bark with good insulating qualities. The thick bark
of most oaks, and especially that of bur oak, makes these
species highly resistant to fire, except when they are very
• After scarring by fire, oaks exhibit high resistance to
rotting. Those of the white oak group produce tyloses, which
are outgrowths of cell walls in response to wounding, which
allow for compartmentalization of fire scar injuries.
• Oaks exhibit deep and wide rooting, giving them better stability.
• Oaks sprout vigorously when young and thus are able to survive
• Fire creates open seed-beds, making germination and survival
of seedlings more effective.
• Acorns germinate in the subsurface of the soil (hypogeal
germination), placing their cotyledons below ground. The cotyledons
of other hardwoods, on the other hand, ascend to the soil
surface. This places the root collars of oak seedlings deeper
in the soil, where they are more protected from fire.
Adaptation of oak leaves and litter to fire
Oak leaves and litter burn much more readily than the litter
and leaves of other hardwoods.
• Oak leaves are much thicker than those of other hardwoods,
giving them greater resistance to decomposition and longer
life spans in the leaf litter.
• Oak leaves tend to be drier (more xerophytic) than other
hardwood species, making them more flammable.
• Oak leaves curl more than other hardwoods. This puts the
fire up off the ground, making it capable or spreading more
effectively. Thus, oak leaves are more flammable and more
capable of “carrying” a fire.
• Oak leaves contain tannins which make them more resistant
to decay, so that it may be several years before all the leaf
material has been turned into compost. Thus, the amount of
burnable material on the oak forest floor is greater than
that with other tree species.
flames from oak leaves in a savanna burn. Note the curled
form of the leaves, which promotes the "carry"
of the fire from one leaf to the next. The temperature
is hot enough to top-kill woody shrubs and trees such
as maple, elm, and walnut, but does not affect the thick
bark of the oak trees.
These characteristics make savanna burns possible. It is
likely that oaks have evolved these leaf characteristics as
part of their evolution as a fire-dependent species. The temperatures
reached in burns is very high, generally more than 300-500
F, but occur only for very brief periods of time. Mineral
soil is a very poor conductor of heat, so that there are few
if any changes in the character of the soil as a result of
a burn. The black seen after a burn is from the ash created
by burned littler. The ashes formed from the burn quickly
Principles of prescribed burns
triangle presents the three components necessary
for initiation of a fire: fuel, oxygen, and heat. Fuel, the
most important factor, consists of organic materials that
are capable of being oxidized. Oxygen is essential for combustion,
since burning is an oxidation process. However, combustion
will not begin unless heat is applied. Heat raises the fuel
to the combustion temperature. At this point, usually around
300-500 F, a fire exists. Once combustion has begun, the combustion
process creates further heat, making it possible for the fire
to propagate itself. Propagation continues until all combustible
fuel is used up, or until conditions change.
The flames of a burning fuel arise from the
combustion of gases which are driven off the burning fuel.
Thus, when one sees visible flames one is seeing an event
resulting from the formation of combustible gases by the burning
The combustibility of the fuel will depend, among other things,
on its moisture content, its physical structure, and its chemical
content. Wet fuel will not burn, or will burn only poorly.
The drier the fuel, the more flammable it will be. Physical
structure refers to the sizes and shapes of the fuel particles.
Large logs burn slower than small logs, which burn slower
than twigs, which burn slower than forbs, which burn slower
than grass. The principle chemical components of wildland
fuels are cellulose and lignin, but some plants also produce
resins or other chemicals that are highly flammable (not common
in Midwest plants).
Dead plant material is more flammable than living plants.
Living plants consist predominantly of water (60-120% or more
moisture content), but once the plant is cut from its roots,
or dies, it begins to lose moisture (curing). The rate at
which moisture is lost will depend on the environmental conditions
as well as upon the physical structure of the plant material.
Under a given weather condition, grasses lose moisture much
more rapidly than shrubs or trees.
Types of fuel
Here we are discussing dead fuels. Because of their high moisture
contents, living fuels are rarely a consideration in savanna
|Grass Dead grasses vary in their flammability, but in general
they are the most flammable fuels in prescribed burns. Tall
grass species such as Indian grass and big bluestem burn hotter
and with higher flames than short grass species such as side
oats grama and little bluestem. The flame height of tall grass
is higher and the rate of movement of the flaming front much
more rapid than that of short grass. Grasses tend to lose
moisture more rapidly than woody plant material (become more
quickly flammable), but they also gain moisture more rapidly.
Forbs Although in arid regions of the United
States many flammable forbs exist, in the Midwest forbs are
relatively fire proof. A forbs-dominated site usually burns
poorly, and does not carry a fire well.
Shrubs Because of their physical structure,
brush and dead shrubs are less combustible than grasses, although
small diameter brush that is very dry can burn rapidly.
Trees Because of their physical structure,
dead trees are less combustible than shrubs, although piles
of dead logs that are very dry (for instance; have been lying
on the ground for some months or years) can be highly combustible.
The flammability of oak leaves was discussed above.
Time lags of fuels
A wet dead fuel will start to dry when the humidity of the
air is less than 100%. The rate at which the fuel dries depends,
among other things, upon its size. Small diameter fuels dry
more rapidly than large diameter fuels. The time lag of a
fuel is expressed as the time it takes for a fuel particle
to reach 63% of its equilibrium moisture content. Time lag
is important in deciding when after major rain events a burn
can be scheduled. Time lag is also important because the flames
themselves will dry out the fuel, and if the time lag is short,
fire may be possible even if the fuel is moderately damp.
Four time-lag classes are recognized:
One-hour time lag fuels are those fuels that
are less than ¼ inch in diameter. These fuels are the most
important for carrying surface fires, and if present their
behavior usually controls fire behavior. One-hour fuels include
oak leaves, grassy fuels, and small twigs. Grasses are the
most important one-hour fuels because they are standing vertically
in the air, where they can equilibrate rapidly.
Ten-hour fuels include small branches and
woody stems, from ¼ to 1 inch diameter. They resist drying
and have a higher heat capacity than one-hour fuels and hence
often do not burn up in low-intensity surface fires. Under
low humidity conditions, however, ten-hour fuels can carry
hot fires and help ignite the larger-sized fuels.
100-hour fuels include larger downed woody
material, with diameters from 1-3 inches. These fuels take
longer to dry and hence combust slowly or not at all under
most conditions. However, if 100-hour fuels do ignite, the
large fuel load means that they can continue to burn for long
periods of time.
1000-hour fuels include large downed branches,
logs, and tree stumps, with diameters greater than 3 inches.
They burn only under prolonged dry conditions or when preheated
by adjacent burning fuels. These 1000-hour fuels can act as
firebreaks during savanna burns, preventing the spread of
the fire across the forest floor. They can also act as fire
shadows, creating unburned areas on the upwind or uphill side
of the fire front. If these fuels do ignite, they can often
burn for days, creating problems in mop-up.
The various categories just mentioned refer to the time lag
before sufficient moisture has dissipated so that combustion
can be initiated. However, it takes different amounts of time
for fuels in these various categories to combust even when
the moisture content is low enough for combustion. This is
because the igniting flame must heat the fuel up to the combustion
temperature, and finer fuels heat up faster than coarser fuels.
Thus, a lighted match may be sufficient to bring dry grass
to the ignition temperature, whereas the same match would
have little effect on a log. This is a matter of compactness
of the fuel as well as its heat capacity. The log has a high
heat capacity and takes a long time to reach its combustion
bramble branches illustrating a 10-hour fuel. These brambles
had been cut the previous winter and were well cured (dried).
The categorization of fuels by time lag implies that they
are all subject to the same environmental conditions. However,
this is almost never the case. Environmental factors that
affect the rate of drying include air temperature, relative
humidity, and wind speed. For instance, a ten-hour fuel subjected
to a 20 mph wind at a temperature of 80 F and relative humidity
of 20% will dry exceedingly rapidly and will become flammable
long before the ten-hour time has been reached.
Flaming and smoldering
There are two kinds of burning. Flaming occurs
when the heat of combustion breaks down the fuel into volatile
combustible products (hydrogen, carbon monoxide, hydrocarbons)
which burn quickly. Flaming produces the least amount of smoke,
and dissipates quickly as the fine fuel is used up. Flaming
provides the visible evidence that a fire exists.
Smoldering occurs when oxygen is limited,
such as when fuels are tightly packed (for instance, a log
pile), in wet fuels, or in fuels that have already been charred.
Smoldering produces the most amount of smoke. Smoldering combustion
may continue for long periods of time on the forest floor,
increasing the possibility of hazard if wind picks up and
carries embers away.
of a smoldering tree. This bur oak, still living, was
hollow and caught on fire from a ground layer burn. The
fire was extinguished with a high-pressure water spray.
It is possible for smoldering combustion to convert into flaming
combustion, or vice versa. If smoldering combustion causes
the breakdown of the fuel into smaller particles, the fuel
may then undergo transition to a state in which it bursts
into flame. Also, often simply adding oxygen (air) to a smoldering
fuel is sufficient to cause it to burst into flames.
On the other hand, flaming combustion can convert into smoldering
combustion if the most flammable fuel elements have been used
up, leaving behind only the heavier, more tightly packed fuel
An understanding of the events leading to transition from
flaming to smoldering or vice versa is essential, especially
in the mop-up phase of savanna burns.
In some savanna burns, fire begins as smoldering and after
sufficient heat has been generated bursts into flaming combustion.
Factors influencing moisture content
of dead fuels
Dead fuels dry by evaporation into the atmosphere. Whether
or not a site will burn will depend strongly on fuel moisture,
which will in turn depend on its previous history, especially
over the last 24-48 hours. Primary weather factors influencing
fuel moisture are sun, wind, rainfall, relative humidity,
and air temperature. Several nice sunny, windy, rainless days
with low humidity and warm temperatures make an ideal lead-in
to a burn.
Slope and aspect also influence fuel drying. Aspect refers
to the direction the terrain is pointing (N, S, E, or W),
and slope refers to the angle of the terrain, both in relation
to the sun. In Midwestern latitudes, steep, south-facing slopes
receive more direct sunlight than flat land and hence dry
Factors influencing flammability of
the fuel in an oak savanna
Flammability can refer to the speed with which a fuel ignites,
the intensity of the heat it produces, and the rate at which
fire spreads. The leaves of oaks burn hotter than those of
any other Midwest forest trees. They carry a fire effectively
because they do not lie flat on the ground, but curl up, so
that they equilibrate more rapidly with the atmosphere. Also,
the curl makes it more likely that fire will pass from one
leaf to the next. (See photo above)
Because of the variability in the oak savanna habitat, burns
of several types can be anticipated. Burns in very open areas
(one tree per acre, for instance) where the dominant fuel
is tall grass, are equivalent to prairie burns. Grass is the
ideal fuel to “carry” a fire, and this is what makes a prairie
burn relatively easy to carry out.
However, grasses vary in flammability. Grasses such as Indian
grass, big and little bluestem, and side oats grama burn extremely
hot, and carry a fire very well. Depending on wind, humidity,
slope, and aspect, flame heights can be 5 feet high or higher.
These prairie grasses will not succeed in shadier areas (30-50%
canopy), where cool season (savanna) grasses often flourish.
Cool-season grasses, ryes and bromes, burn cooler and do not
carry a fire nearly as well.
The principal fuel in shadier areas of an oak savanna are
oak leaves, which are vital to the burn. Oak leaves are admirably
suited for fire (see above) and it is undoubtedly because
of their nature that the oak savanna habitat exists.
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